1. Field of Invention
The present invention relates to methods and apparatuses for supporting a workpiece, and also relates to methods and apparatuses for heat-treating a workpiece.
2. Description of Related Art
Numerous applications require a workpiece to be supported. For example, in the manufacture of semiconductor chips such as microprocessors, the workpiece typically includes a semiconductor wafer, which must be supported in a thermal processing chamber for annealing or other heat-treating processes. The wafer is normally supported in the chamber by either a plurality of support pins (typically quartz), or alternatively, by a guard ring (typically formed of a semiconductor material similar to that of the wafer itself). Due to the fact that physical contact between the support pins or ring can cause thermal gradients in the wafer during thermal processing, thereby damaging either the crystal lattice of the wafer or devices in the wafer, the pins or guard ring in many typical systems contact the wafer only at its outer edges, within the confines of a narrow “exclusion zone” or waste zone, which is not used to produce semiconductor chips. The exclusion zone extends radially inward for only a small distance (e.g. 3 mm) from the outer circumferential edge of the wafer. However, the support pins in at least some conventional systems contact the wafer at locations other than the exclusion zone. For example, one conventional system employs contact pins that contact the wafer at a distance from the center of the wafer of about two-thirds of the wafer's radius. Thus, although use of the exclusion zone to support the wafer is advantageous for many applications, it is not viewed as essential.
Device size and performance requirements of semiconductor chips are becoming increasingly demanding, with the result that new heat-treating methods are emerging, and existing heat-treating methods are being modified, to attempt to satisfy these evolving requirements. However, conventional methods of supporting the semiconductor wafer may no longer be appropriate for some specific applications of these emerging heat-treating methods.
Examples of such emerging heat-treating methods are disclosed in commonly-owned U.S. Pat. No. 6,594,446 which issued to Camm et al, on Jul. 15, 2003, and commonly-owned U.S. patent application Ser. No. 10/005,186 published on Aug. 1, 2002 as Publication No. US 2002/0102098, both of which are hereby incorporated herein by reference. One such method involves pre-heating the wafer to an intermediate temperature at a rate that is slower than a thermal conduction time through the wafer, so that the entire wafer is heated relatively uniformly to the intermediate temperature. This pre-heating stage may be achieved by irradiating the back-side or substrate side of the wafer with an arc lamp, to heat the wafer at a ramp rate such as 100° C. per second to 400° C. per second, for example. Following the pre-heating stage, the top-side or device side of the wafer is rapidly heated to a substantially higher annealing temperature, at a rate much faster than a thermal conduction time through the wafer, so that only the top-side surface region of the wafer is heated to the final annealing temperature, while the bulk of the wafer remains close to the relatively cooler intermediate temperature. This may be achieved by exposing the top-side surface to a high-power flash from a flash-lamp, for a relatively short duration, on the order of one millisecond, for example. The cooler bulk of the wafer then acts as a heat sink to facilitate rapid cooling of the top-side surface.
Other annealing methods, which do not necessarily involve any pre-heating stage, include rapidly heating the device side surface of the wafer to the annealing temperature using excimer lasers or a microwave pulse, for example, while the bulk of the wafer remains at a much lower temperature. One variation of such a method involves scanning a laser line across the surface of the workpiece, to rapidly heat the area of the workpiece covered by the line as it scans.
However, the present inventors have discovered that certain specific implementations of such annealing methods may give rise to special problems and difficulties that have not been previously discovered or appreciated. In particular, the present inventors have discovered that conventional methods of supporting a workpiece may not be suitable for some applications of these annealing methods, as discussed in greater detail herein.
Accordingly, there is a need for an improved method and system for supporting a workpiece. There is also a need for an improved method of heat-treating a workpiece which assists in addressing the difficulties discussed herein.